Ibrutinib, an inhibitor of Bruton´s tyrosine kinase (BTK) is approved in CLL as continuous, daily administration of 420 mg orally until progression. Ibrutinib drug costs in health care are rapidly increasing and are difficult to predict, as long-term follow up analyses have shown that many patients remain on therapy for several years, in some cases even many years. It has been observed that patients who stop ibrutinib due to side effects may often remain with continued CLL disease control i.e. in stable partial remission even when off ibrutinib therapy. There are also emerging data on mutations within BTK, with loss of efficacy of ibrutinib, during long-term continuous administration. These observations raise the question whether alternative dosing strategies may be feasible. This pilot study will explore intermittent and repeated dosing of ibrutinib, until alternative therapy is required due to resistance or intolerance to ibrutinib. An ON-OFF dosing strategy will be applied, where advanced-phase CLL patients who have received at least 6 months of ibrutinib and who have achieved a stable PR will stop ibrutinib and be followed off therapy until clinical progression, at which ibrutinib will be re-instituted. Such ON-OFF ibrutinib cycles may be repeated until non-tolerability or resistance, or need of continuous dosing of ibrutinib (i.e. early progression when off the drug). If successful, the study will indicate a way forward towards reducing ibrutinib drug costs in health care without affecting long-term disease control, possibly also with fewer ibrutinib-related side effects due to a lower cumulative dose of ibrutinib. Long-term effects on potential mutations within BTK and its downstream signaling molecules will also be analysed.